P–T–t conditions of Early Palaeozoic low-P high-T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam

IF 3.5 2区 地球科学 Q1 GEOLOGY
Nam Nguyen Duc, Ching-Hua Lo, Tadashi Usuki, Yoshiyuki Iizuka, Pham Binh
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引用次数: 0

Abstract

High-grade metamorphic rocks are widely exposed along the SE–NW- to E–W-trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high-grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse-grained granulite mineral assemblages, defining a tight clockwise P–T path with near-isobaric heating to a near ultrahigh-temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse-grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K-feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post-peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium-pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine-grained kyanite-bearing mineral associations. Petrographic observations indicate that these fine-grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post-collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

越南中部张松带南部早古生代低磷高钾麻粒岩相变质作用的P - T - T条件
高品位变质岩广泛暴露在越南中部Truong Son带的SE–NW至E–W走向剪切带上,但在该地区进行的岩石学研究很少。在此,我们报道了在Truong Son带最南端的Dai-Loc剪切带上出现的糜棱岩化麻粒岩。对两个麻粒岩样品的演化进行了详细的岩相分析、地球化学和P–T–T估计,以阐明这些高级变质岩的形成过程。结果表明,这些岩石经历了两个不同的变质旋回。第一个循环(M1)以粗粒麻粒岩矿物组合为特征,定义了一条紧顺时针P–T路径,在低压下近等压加热至近超高温峰值,然后冷却。前进矿物组合(M1a)由堇青石包裹体指示 + 硅线石 + 黑云母 + 石英 + 尖晶石 ± 粗粒石榴石、斜方辉石和堇青石中的斜长石。石榴石的矿物组合 + 斜方辉石 + 堇青石 + 斜长石 + 钾长石 + 钛铁矿 + 熔化 ± 黑云母(M1b)定义了5.3–6.3 kbar和850–920°C的峰值P–T条件。峰后冷却(M1c)以石英的形成为标志 + 黑云母在石榴石和斜方辉石周围呈同向分布。第二个旋回涉及中压角闪岩相变质作用(M2),其特征是细粒蓝晶石矿物组合的区域发育。岩石学观察表明,这些细粒组合是在糜棱岩化过程中形成的。锆石U–Pb定年显示,麻粒岩相变质作用的时间似乎与430–410碰撞后花岗质岩石的侵入同时发生 麻粒岩相变质作用和地壳熔融可能是由早古生代板块断裂引发的软流圈地幔上升流所驱动的。考虑到以往的构造和地质年代研究,第二次变质事件可能发生在三叠纪-印支造山运动期间。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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